Over time sailing vessels' hull design has evolved from ‘displacement’ hulls with a fixed maximum boat speed consequent on hull being ‘pushed’ through the water to planing hulls. Planing hulls achieve significantly higher forward velocity by sailing atop or ‘planing’ on water thereby reducing resistance or drag. Consequent on this increased boat speed was the realisation by naval architects that a keeled sailboat's speed could exceed the wind's velocity. This phenomenon only occurs when vessel is sailed ‘off the wind’ (as in vessel is aligned at greater than 90° to the direction of the wind). This ‘off the wind’ attitude is in contrast to a ‘close-hauled’ attitude, wherein vessel is aligned at less than 90° to the wind's direction.
A further consequence of this ability to exceed wind speed when sailing ‘off the wind’ being that such vessel would therefore never sail directly downwind as their sails would collapse when vessel's speed attains wind speed (as in ‘running’ with wind—vessel orientated 180° to wind's direction). Modern sailing vessels therefore always sail either ‘off the wind’ or ‘close-hauled’.
Both these attitudes result in the vessel's hull ‘heeling’ secondary to the force generated by the wind on the sails. Control of vessel's ‘heeling’ moment is therefore required.